The present disclosure relates generally to communication, and, amongst other things, to transmitters used in communication systems.
Communication systems transport information between two points, for example, wirelessly. A base station communicates wirelessly with one or more access terminals to transport data, voice or other information. The term base station can be used interchangeably with access point or access terminal, and the term access terminal can be used interchangeably with wireless communication device, user equipment, handset, or mobile station or unit.
In a wireless communication system, an RF modulated signal from a transmitter may reach a receiver via a number of propagation paths. The characteristics of the propagation paths typically vary over time due to a number of factors such as fading and multipath. To provide diversity against deleterious path effects and improve performance, multiple transmit and receive antennas may be used. A multiple-input multiple-output (MIMO) communication system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, with NS≦min {NT, NR}. Each of the NS independent channels may also be referred to as a spatial subchannel (or a transmission channel) of the MIMO channel.
An orthogonal frequency division multiple access (OFDMA) system is a multi-access system that utilizes OFDM. OFDM is a multi-carrier modulation technique that partitions the overall system bandwidth into multiple orthogonal frequency subcarriers. An OFDMA system may use time and/or frequency division multiplexing to achieve orthogonality among multiple data transmissions for multiple access terminals. For example, different terminals may be allocated different subbands, and the data transmission for each terminal may be sent on the subband(s) allocated to this terminal. By using disjoint or non-overlapping subbands for different terminals, interference among the multiple terminals may be avoided or reduced, and improved performance may be achieved. The number of non-overlapping subbands available for data transmission is limited (to K) by the OFDM structure used for the OFDMA system. The limited number of subbands places an upper limit on the number of terminals that may transmit and/or receive simultaneously without interfering one another.
Often, transmission signals exhibit a high peak-to-average ratio (PAR). This can cause distortion when passed through non-linear power-amplifiers that are commonly found in transmitters. Conventional techniques reduce the PAR using clipping or back-off of the time-domain transmission waveform to operate in the linear region, which wastes power because power-amplifiers are more efficient when operated in the non-linear region. Other techniques consume bandwidth by injecting an additional counter-waveform that is summed with the transmission waveform to reduce the peaks, but this consumes bandwidth. One such technique uses tone reservation where one tone is reserved to use as a counter signal to reduce the PAR.